In an internal combustion engine used for vehicles, there are some types in which a throttle valve (restriction valve) is provided on the upstream side of an air intake manifold (air intake path), and a fuel injection valve and an air flow rate sensor are provided on the upstream side thereof or on the downstream side thereof (For example, refer to Japanese Published Examined Patent Application, Second Publication No. H04-15388 (Page 2, FIG. 1))
The air flow rate sensor detects the amount of intake air and outputs an intake air rate signal. In a case where the sensor is provided on the downstream side of the throttle valve, generally, it is installed in the axis direction of an air intake path of the air intake manifold.
The intake air rate signal outputted by the air flow rate sensor is inputted into a control circuit, and a fuel injection amount is calculated in response to the running state of the engine. A fuel injection amount signal based on the calculated fuel injection amount is outputted from the control circuit, and the operation control of the fuel injection valve is performed.
In an air intake device of a conventional art internal combustion engine, the air flow rate sensor is an air flow meter that detects the amount of air as a mass flow amount. In a case where the air flow meter is installed on the upstream side of a throttle valve, since the air flow meter measures the total of an amount of air suctioned into a combustion chamber and an amount of air filled in an air intake manifold, there is a problem in that the amount of air suctioned into the combustion chamber cannot be accurately measured in the period of transition.
On the other hand, in a case where the air flow meter is installed on the downstream side of the throttle valve in the axis direction of an air intake path, it is not possible to accurately measure the amount of intake air due to disturbance of air, which occurs on the downstream side of the throttle valve. That is, one end of the throttle valve is inclined in the forward direction in regard to the streams of air by rotation of the throttle valve, and simultaneously, the other end of the throttle valve is inclined in the reverse direction of the streams of air. In a case where the throttle valve is slightly opened, since a gap is formed between the throttle valve in the air intake path and the air intake path, air is caused to flow to the downstream side of the throttle valve along the inner wall in the air intake path through the gap.
Therefore, air straightly flows along the inner wall of the air intake path on the downstream side at one end side of the throttle valve inclined in the forward direction, and air flows while forming a vortex toward the center direction of the air intake path on the downstream side at the other end side of the throttle valve inclined in the reverse direction. That is, an air flow that is substantially parallel to the inclination of the throttle valve is formed from the other end side to one end side on the downstream side of the throttle valve. Therefore, if the air flow meter is installed immediately behind the throttle valve so as to extend in the axial direction of the air intake path, there is a problem in that the air flow meter cannot accurately measure the air amount suctioned into the combustion chamber.
Further, when the throttle valve is fully opened, although air flows in the air intake path, since an area in which the velocity of air becomes slow is brought about on the downstream side of the turning shaft of the throttle valve, there is a problem in that the air flow meter cannot accurately measure the amount of air suctioned into the combustion chamber if the air flow meter is installed immediately behind the throttle valve so as to extend in the axis direction of the air intake path.
In order to solve these problems, it is necessary that, in the prior arts, an air intake path is made larger to reduce disturbance of air, and the air flow meter is installed at a suitable position.
The present invention was developed in view of such situations, and it is therefore an object of the invention to provide an air intake device for an internal combustion engine and a method of measuring an intake air amount, which are able to accurately measure the air flow rate and are able to downsize the air intake path.